The invention relates to an illumination device comprising:
a housing, PA1 a lamp holder and a concave reflector having an optical axis in the housing; PA1 an electric lamp comprising a light-diffusing lamp vessel and light source inside the lamp vessel, arranged inside the reflector and substantially coaxial with the reflector; PA1 a lamp cap securely connected to the lamp vessel and accommodated in the lamp holder.
The invention also relates to a capped lamp/reflector unit suitable for use in this device.
Such a device is known from EP 0 168 016A. The known device is designed for use as a portable mine illumination device. Its electric lamp has an outer bulb which scatters light just as, or instead of the lamp vessel.
Without a light-scattering lamp vessel and/or outer bulb, the lamp in conjunction with the reflector would give a narrow light beam with a high intensity in the beam centre and a quick intensity decrease at small angles to the centre. If an envelope having a light-scattering surface obtained by sandblasting or chemical etching is used, the intensity in the beam centre is much lower and the intensity initially decreases slowly from the centre, and later more quickly. The beam as a result is much wider. A larger surface can be illuminated with the beam, but less brightly. A further intensification of these effects is obtained through the use of a second light-scattering envelope.
A disadvantage of an envelope frosted by sandblasting, and similarly of such an envelope frosted chemically, is that the frosting causes some reduction of the luminous flux, approximately 5 to 6 percents, as a result of light absorption.
Light-scattering means are also used in commercially available illumination devices of the construction described and designed for making video registrations. Thus, devices are known in which the luminous window of the reflector is covered by a frosted pane; other devices have reflectors whose concave surface is rough.
Light-scattering means not only yield a wider beam of a lower intensity in the centre thereof and a gradual decrease of this intensity outside the centre, but also a greater rotational symmetry of the beam. Deviations in the rotational symmetry of the lamp itself, for example, owing to a current supply conductor which runs alongside the light source, are reduced by such means, while the evenness of the beam is increased. A screen illuminated with such a beam has an illuminated field which is to a considerable degree rotationally symmetrical. The illuminated field has an illuminance which is very even, not only as regards rotational symmetry, but also diametrically: a comparatively high illuminance in the centre and a gradual decrease therefrom towards the outer edge.
The known illumination devices, however, have the disadvantage that video, film, or photographic shots of a poor illumination quality are obtained when these devices are used, in spite of the evenness of the beam formed by these devices and the even illumination of the field covered by this beam.